This invention relates to a process for the recovery of valuable components from the off-gases of a urea granulation system as well as from the liquid waste streams obtained in the preparation of urea.
In the preparation of urea from ammonia and carbon dioxide, a high temperature and correspondingly high pressure are used to form a urea synthesis solution which, in addition to product urea, still contains a considerable amount of free ammonia and unconverted ammonium carbamate. This urea synthesis solution is thereafter treated in one or more stages to decompose the ammonium carbamate into ammonia and carbon dioxide which are then mostly removed from the solution, together with the free ammonia present, and are usually recycled. From the last carbamate decomposition stage, an aqueous urea solution is obtained which still contains some dissolved ammonia and carbon dioxide, which are subsequently removed by expansion of the urea product stream to atmospheric or lower pressure. The resulting aqueous urea solution is then concentrated by evaporation and/or crystallization and further processed.
During this evaporation and crystallization, a gas mixture is formed which, in addition to water vapor, contains entrained fine urea droplets, as well as ammonia and carbon dioxide. This gas mixture is condensed, and the resulting condensate, together with the condensate formed from the gas mixture separated in the expansion of the urea solution after the last decomposition stage, becomes what is called process condensate. Part of this process condensate is returned into the process for the absorption of the gas mixture discharged from the last ammonium carbamate decomposition stage. The remaining portion of this process condensate is generally drained off or discharged from the process.
The process condensate also includes water initially introduced into the process as steam for the operation of the ejectors in the evaporation and/or crystallization section, wash water, rinsing water on the packing glands of the carbamate pumps and the like. Furthermore, for each mole of urea produced, one mole of water is formed. This means that, in a urea plant having a capacity of 1000 tons of urea per day, 300 tons of water will be formed in the synthesis. In addition, depending upon the temperature of cooling water used, approximately 200 to 315 tons per day of water are introduced into the process, so that in total roughly 500-615 tons of water may have to be discharged from the process per day.
This process condensate generally contains about 2 to 9 percent by weight ammonia, 0.8 to 6 percent by weight carbon dioxide, and 0.3 to 1.5 percent by weight urea. To simply discharge these materials from the process with the process condensate represents, on one hand, a loss of a substantial quantity of raw materials. On the other hand, this represents a substantial load to the surface waters into which this waste water would be discharged, and is no longer permitted in many countries.
If the urea product is intended for use as a fertilizer, then the further processing after the evaporation or crystallization step as a rule consists of granulation. A frequently used method of granulation is prilling, wherein a virtually anhydrous melt of urea is sprayed into a prilling tower in counterflow with a cooling gas, generally air. The virtually anhydrous melt can be obtained either by evaporation of aqueous urea solutions, or by melting urea crystals. Another known method for the preparation of urea granules is the spraying of highly concentrated urea solutions or urea melts onto fine urea particles or seeds maintained in a fluidized state by air.
In either of these techniques, large amounts of air are discharged from the granulation system, in which fine liquid and/or solid urea particles are suspended. In a urea plant having a capacity of 1000 tons of urea per day, this air contains approximately 30 to 40 kg urea dust per hour if the granulation by prilling, and can even be approximately 80 kg urea dust per hour when the fluid-bed granulation technique is applied. If this air were to be simply vented to the atmosphere, these amounts of urea dust would constitute a substantial loss of valuable product, and moreover would represent a degree of air pollution which is neither desirable nor permitted in many countries.
Methods are already known for removing the greater part of the ammonia and urea present in process condensate streams before such process condensate is discharged to the environment. One such method is described in Industrial Wastes, September/October, 1976, pages 44-47, wherein process condensate obtained in a urea synthesis plant, which has already been freed of a portion of this ammonia and carbon dioxide by desorption at a low pressure, is introduced into the bottom of a reaction column at a higher pressure wherein it is heated by means of steam resulting in the hydrolysis of urea. The solution thus obtained, containing ammonia and carbon dioxide, together with a small amount of non-hydrolyzed urea, is removed from the top of the reaction column, and expanded to the aforementioned low pressure whereupon the ammonia and carbon dioxide are removed in a second desorption column by stripping with steam. The gas mixture removed from the second desorption column can be used as a stripping medium in the first desorption stage.
The bottom product from the second desorption column is discharged to waste after it is used to heat a further portion of process condensate to be treated. Under practical conditions, however, this waste stream will still contain approximately 50 ppm ammonia and 50 ppm urea. Even after very long residence times, for which inefficiently large reaction columns would be required, it is not possible to achieve urea and ammonia contents in accordance with this method of less than 20 to 25 ppm.
In an improved process for the treatment of process condensate disclosed in copending application Ser. No. 325,922, filed Nov. 30, 1981, and now U.S. Pat. No. 4,456,535 the hydrolysis of the urea contained in the process condensate is carried out in counterflow with an inert gas, preferably steam, in a reaction column wherein the bottom temperature is maintained at about 180.degree. to 230.degree. C., and the top temperature is maintained at about 170.degree. to 220.degree. C. In this manner, the ammonia and urea contents of the residual waste liquid stream can be reduced to a level of 10 ppm or less.
However, neither the above-noted article nor patent application contain any suggestion of means to remove the pollutants from the air stream resulting from urea granulation.
One known method for treating air which is discharged from a urea granulation process is disclosed in British Pat. No. 1,528,051. In this process, the air discharged from the granulation process, containing entrained urea particles is introduced into the bottom of a washing column wherein it is passed countercurrently against a dilute aqueous urea solution which has been obtained by the condensation of vapors from the urea solution evaporation stage. In this manner, the urea particles entrained in the air stream are washed out and dissolved in the aqueous urea solution. The urea solution discharged from the bottom of the washing column is led back into the first evaporation stage. By reason of the heat exchange between the hot air from the granulation system and the wash liquid in the washing column, water is evaporated so that a mixture of air and water vapor is discharged and vented from the top of the washing column.
This known process has the disadvantage that any pollutants present in the urea used for the granulation are washed out in the washing column and end up in the urea solution to be evaporated. Moreover, any additives which may have been added to the urea melt prior to granulation, such as formaldehyde or formaldehyde derivatives, will also end up in the evaporation section via the wash liquid, and these additives can impede the evaporation process by the formation of foam. If it is desired that the urea solution recycled to the evaporation section is fairly concentrated, for instance 20 to 25 weight percent urea, then the wash liquid must be recirculated over the washing column which presents the danger of entrainment of droplets of fairly concentrated urea solution in the gas stream, resulting in a considerably greater loss of urea than when the washing solution need not be recirculated. Moreover, if glass fibers are used as a packing material in the washing column, there is a real danger of attack on this packing material resulting from the high pH of the solution due to the presence of ammonia and ammonium cyanate formed in the hydrolysis of urea.